1. Field of Invention
This invention relates to the growth of doped semiconductor crystals and, in particular, the doping of a melt as part of the process of growing the doped semiconductor crystals.
2. Description of Related Art
Semiconductor wafers can be cut from semiconductor crystals grown in a Czochralski-type crystal-growing furnace. The semiconductor wafers typically include a controlled concentration of a dopant to produce desired electrical characteristics. The dopants are typically added to a melt such as, for example, molten silicon, in the crystal-growing furnace. The silicon, for example, and the dopant mix together in a liquid state to produce a molten mixture having the desired dopant concentration. A single-crystal ingot is pulled from the molten mix.
Some dopants, such as antimony, have a vapor pressure sufficiently high to cause the concentration of dopant in the melt to change significantly as the silicon, for example, and dopant are melting together in the furnace. The change in dopant concentration cannot be accurately predicted, so the ability to accurately produce crystals having desired electrical properties is limited. To overcome this problem, several techniques have been developed for adding high vapor pressure dopants to the semiconductor after it is melted. Such techniques are difficult to perform, however, because the molten semiconductor (the "melt") must be maintained in an inert atmosphere in the crystal-growing furnace.
A Czochralski-type crystal-growing furnace typically includes two separately sealed vacuum-tight chambers. The upper chamber, or pull chamber, has space for enclosing the ingot as it is grown and includes a seed cable or shaft for lowering and raising a seed crystal. The lower chamber, or furnace tank, includes a crucible containing the melt.
Several methods for adding the dopant to the melt are known. For example, the melt can be doped by attaching a dopant to a seed crystal and then dipping the seed and dopant into the melt. However, dipping the seed into the melt, without appropriate preheating, causes thermal stress within the seed, which can result in cracks or structural defects. Preheating of the seed may cause premature dopant drop, resulting in a melt splash. Various methods for attaching the dopant to the seed so as to prevent premature dopant drop are known. One such method includes boring a transverse hole into the seed crystal, melting the dopant and then solidifying the dopant around the seed and in the hole. Alternatively, a ring of dopant can be placed over the seed and kept from dropping off by laser-welding a block of silicon to the bottom of the seed.
U.S. Pat. No. 5,406,905 shows a method of doping the melt by attaching the dopant to the seed crystal and then lowering the dopant-seed assembly to just above the melt and holding it there while heat from the melt heats the seed and the dopant, allowing the dopant to slip off of the seed.
It is also known to form the dopant into a thin electrical current-carrying wire attached to the seed crystal. The seed contacts the molten semiconductor, causing the dopant wire to melt and thereby interrupt the current flow. The interrupted current flow automatically causes a computer to begin the crystal-pulling procedure.